This invention relates to optical tracking systems and more particularly to an automatic precision laser tracking system for aircraft and the like.
An automatic tracking system to which this invention is directed utilizes a pulsed laser as an energy transmitter and, in addition to determining range by timing the return of reflected pulses as in a radar unit, measures changes in the position of the axis of the stream or beam of return pulses relative to a stationary reference element. In this manner error signals of the proper magnitude and sense are generated for causing the transmitted pulse train to track the target. A typical reference element is an optical sensor such as a silicon photodiode which is symmetrically segmented in quadrants about a central axis. When the axis of the return pulse train coincides with the axis of symmetry of the photodiode, the energy outputs from the diode segments are equal and no error signal is produced. When the incident pulse train axis of offset from the diode axis, the diode segment outputs are unequal and constitute an error signal of the proper magnitude and sense from which a servo drive signal is derived for keeping the transmitter pulses on target. The servo system may drive mirrors for this purpose.
A difficulty with this tracking system is spurious error signals from scintillation of the optical beam as a result of atmospheric turbulence. The turbulence consists of fluctuations in the refractive index of the air which are caused by temperature variations and gradients. Such scintillation causes the diode segments to produce significant output error signals when the beam and diode axes are coincident and results in inaccurate tracking data as well as instability, jitter and oscillations in the servo system. Such errors are especially critical when the optical tracking system is used to calibrate instruments in the aircraft and ground stations.
An object of this invention is the provision of an optical tracking system in which scintillation of the laser beam is automatically compensated.
Another object is the provision of a precision automatic tracking system with the combination of an energy distribution sensor for producing tracking error signals and an optical diffusing medium for essentially eliminating time-variant energy distribution in the laser beam.